1. Field of the Invention:
This invention relates to single lens reflex cameras or video cameras of the interchangeable lens type and, more particularly, to a camera system suited to perform an automatic focus adjustment with good efficiency.
2. Description of the Related Art:
Recently, the employment of focus detecting devices of the TTL type in single lens reflex cameras is rapidly spreading. Even a camera having automatic focus adjustment, if of the single lens reflex type at all, is required to have the capability of using photographic supplementary means such as extenders for enlarging the focal length of the photographic lens, reducers for contracting the focal length or like converters, or close-up lenses.
Meanwhile, in order to realize high accuracy of focus adjustment, the focus detecting device repeats a periodic focus detection during the time when the focusing operation (movement of the focusing lens) is controlled in accordance with the preceding detection result. Therefore, the focusing speed and the period of focus detection are in such a precisely-timed relationship that one cycle or a few cycles of detection suffice for realizing a sharp image plane in the depth of detection.
However, suppose a converter of 2.times. magnification is attached between the photographic lens and the camera body to increase the focal length 2 times, the relation between the defocus amount the camera detects and the amount of focusing movement is caused to change at a rate of the square of the magnification of the converter. If no measure is taken in determining the required amount of movement of the lens, it will result that, as the focusing lens moves the determined distance, the sharp image plane shifts 4 times more than the correct distance. Thus, the sharp image plane overruns the in-focus position, the overdue distance being reflected to a 3 times larger defocus than the previous one. If such driving proceeds, the adjusting action diverges, and a sharp focus becomes eventually impossible to attain. To avoid this, the camera may be provided with means for recognizing electrically or mechanically the presence or absence of a converter to slow down the focusing speed to 1/4 of the normal value. Yet, as has been proposed in U.S. patent application Ser. No. 913,343 (Japanese Patent Application No. Sho 60-219521), with a view to a higher accuracy of focusing control, merely accounting for the magification of the converter is not always sufficient. In addition, it is found better that a correction factor concerning the specific performance of the lens is introduced.
Aside from this, the compensation tendency to the aberration correction of the photographic lens and the spectral sensitivity characteristic of the focus detection device in some combinations give rise to a discrepancy between the best focal position for the photographic lens and that determined to be best by the focus detecting device. So, it is preferred to take this information into account when the focusing position is adjusted. Also, vignetting information of the combined lens system, too, must be input in accompaniment with the magnification information of the extender.
FIG. 1 in block form illustrates the flow of signals representative of such correction factors of the focusing speed when an extender is attached. The photographic lens 1, the extender 3 and the camera body 5 have respective units of electrical circuit elements. Information from a memory device 2 such as ROM in the photographic lens 1 and information 4 representative of the use of the extender 3 are supplied to a conversion circuit 6 in the camera body 5, of which the output is applied to a microcomputer (processor) in the focus detecting device 7 where the required values of the correction factors are computed.
However, because some of the information cannot be linearly converted, for all information treated in the camera body, a list of the values of the correction factors of all interchangeable lenses which have so far been available to the camera body and another list of the values of those of all converters usable with the interchangeable lenses should be prepared in the form of a table. Preparing such a table is, however, actually very difficult to do. Moreover, there arises a problem that information pertaining to any interchangeable lenses developed after the purchase of the camera body will not be included in the table as to the technique of communicating between the camera body and the interchangeable lens reference is made to U.S. patent application Ser. Nos. 688,355 and 829,792, now U.S. Pat. No. 4,687,915.